Owing to external interference, disturbing voltages may be induced between circuits which are separated by means of an impedance. Examples of such separated circuits are the electronic control circuits in telephone exchanges on the one hand and the electronic circuits which are directly connected to the telephone lines on the other hand. The conductors of these lines may have a high common-mode voltage with respect to the central battery of the telephone exchange due to inductive interference by high-voltage power circuits or due to lighting, this interference acting in the longitudinal direction in these conductors. Disturbing voltages may also be formed by common-mode speech signals in the telephone circuits or by common-mode charging signals on the subscriber's lines.
Generally, it is a requirement that the voltages between the voltage reference points of the separated circuits do not affect the transmission of signals from one circuit to the other.
It is known to use transformers or other types of galvanically separated signal transmission such as, for example, optoelectric coupling devices, for the signal transmission. Such solutions are expensive or are only linear in a limited amplitude range.